Electric current that reverses periodically, usually many times per second, is called “alternating current.” In most public or commercial power distribution networks, electrical current is delivered to a customer or user as alternating current.
Electric current is induced to reverse or alternate periodically by a voltage that reverses or alternates periodically. One complete voltage period, with accompanying current flow in one direction and then the other, is called a cycle. In the United States, 60 cycles per second (also referred to as “60 Hz”) is the standard frequency of alternating current in most environments. In Europe, 50 Hz is the standard.
Typically, the graphical form of the level of alternating voltage over a period or cycle is generally sinusoidal. This is because the near sinusoidal form is relatively easy, economical, and efficient to generate, deliver, and utilize.
Power distributed to small businesses or homes is commonly “single phase” or “dual phase” power. In a single phase system, a single alternating voltage is distributed through a two-line connection. In a dual phase system, two alternating voltages are distributed through at least three lines: one neutral line and one other line for each of the two alternating voltages. The time that the voltage on one of these lines is zero and the time that voltage on the second of these lines is zero are separated by a time period equivalent to the time lapse of one half of one cycle. The two voltages are separated in time by a “phase difference”—that is, the sinusoidal form of the voltage on one line leads or lags the sinusoidal form of the voltage on the other line by the amount of the phase differential. The effective voltage between the first phase line and the second phase line is therefore significantly greater that the effective voltage between each of the phase lines and the neutral line. As a result, a three-line, two-phase system may provide, for example, 120 volts in a phase-to-neutral line circuit and 240 volts in a phase-to-phase line circuit.
In large commercial and industrial applications, three phase systems have long been common. In three phase systems, each voltage cycle on each phase line is 120 degrees, or ⅓ of a period, out of phase with the voltage cycle on each of the other two phase lines. Three phase systems are used in large commercial and industrial applications because three-phase equipment is smaller in size, weighs less, and is more efficient than single or dual phase equipment. Although three phase circuits are somewhat more complex than single or dual phase circuits, they too weigh less than single phase circuitry for the same loads supported by the circuitry. Three phase circuits also can provide a wide range of voltages and can be used for single or dual phase loads.
Three phase circuits power is generated by circuits in either of two configurations: (i) a “delta”; or (ii) a “wye” configuration. If one end of each of the legs of a three-phase circuit are centrally connected at a common point and the other ends are connected to three phase lines (one line for each phase), the configuration is called a wye or “Y” connection. If the legs of the three phase circuits are connected instead in series to form a closed loop, with one phase line connected to each junction of two adjacent legs, the configuration is called a delta or “Δ.”
One reason that three phase circuits are more complex than typical single phase and dual phase circuits is the need to maintain at least somewhat balanced loads among each of the three phases. One indicator of imbalance is the level of current flowing through each phase line. If the level of current flowing through a phase line is different than that flowing through a different phase line, the load is obviously unbalanced. In a wye connected system, imbalance can also be indicated by current flowing through the neutral line, and this situation can arise when the amount of current flowing through each phase is identical in amplitude but differing in phase due to the nature of the loads served by the lines. Imbalance between the loads can result in damage to the three phase system, can cause excessive wear of components in the system such as the three-phase generator, and can be difficult and costly to correct.
For example, in many industrial three-phase applications, such as computer and communications network applications, three-phase power is supplied to racks of equipment. One common prior art system provides three-phase power to one or more racks via a four line input, providing a line for each voltage phase and a common ground or neutral line. An elongated power distribution plug strip connects to the input and distributes power of differing phases to a plurality of plug strip outputs for the phase. The three-phase plugstrip typically provides three branches of outputs, one branch for each phase of power provided by the three-phase plugstrip. This plugstrip is mountable on or adjacent to a given equipment rack in order to supply three branches single phase power (with each such branch derived from the three-phase power intput) to the rack or other equipment in the vicinity.
In order to help ensure that each branch of outputs supplies a cumulative load that is balanced as compared to the cumulative load served by the other branches of outputs, this prior art three-phase plugstrip has included a single current display visible to an operator along the face of the plugstrip in which the outputs are also mounted. The installer or on-site operator or user (collectively the “operator”) can use this display to determine the total amount of current flowing through each branch of outputs by physically pushing an associated button mounted on the plugstrip housing. This causes the display to cycle through numerical indications of the total current for each differing phase branch of output. If the current indicated for one phase set (branch) is different than that for another current indicated for a different phase set of outputs, the loads are unbalanced. When this difference becomes too great, the operator is thereby alerted to the need to take corrective action to correct the imbalance.
The applicant has discovered that another problem with this type of prior art system is that it requires the operator to take the time and effort to stop whatever the operator may be doing, turn attention to the plugstrip, and press the display button on the plugstrip to cycle through and observe the current indicators for the various phases. In the case of a wye connected system, this type of prior art also typically has not provided for display or ready determination of the level of current flowing through the neutral line. As a result, users of such prior art systems may not be receive any indication of imbalance when current flows through the neutral line, indicating an imbalance in fact, but the current level flowing through each phase provides no such indication.
Prior art power supply systems have also included other tools to help determine imbalance among loads supplied by three-phase systems. One such system in common use includes a remote power management feature that remotely monitors the power supply plugstrip through a network connection between the plugstrip and a computer with an associated computer screen. A user at the computer screen can observe information about each phase of power and whether there is an imbalance between phases.
This type of network system, however, requires the user to have access to a computer screen in order to observe information about the level of imbalance, and this type of computer screen is often unavailable or is otherwise inconvenient to inspect at the on-site location of the plugstrip itself. At this location, for example, there may be no space available for a computer screen much less one networked to the plugstrip, and during installation of the plugstrip, the installer may have little time or ability to set-up or inspect a computer screen.